GnRH is a decapeptide produced by hypothalamic neurons and secreted into the hypophysioportal circulation via portal vessels. It is first synthesized as a larger precursor protein which is processed by proteolytic cleavage and amidation at its C-terminal glycine. GnRH stimulates gonadotroph cells in the anterior pituitary gland to release luteinizing hormone and follicle-stimulating hormone, thereby regulating the hypothalamic-pituitary gonadal control of human reproduction.
The involvement of GnRH has been implicated in certain carcinomas, and GnRH analogues have been used in the treatment of breast, prostatic, pancreatic, endometrial and ovarian cancers (Kadar et al., 1988, Prostate 12:229-307). The analogues suppressed tumor cell growth in vitro and in vivo. In addition, GnRH binding sites have been reported in certain solid tumors and in established cell lines (Emons et al., 1993, J. Clin. Endocrinol. Metab. 77:1458-1464), though preliminary results suggest that the GnRH receptor (GnRHR) involved might differ from the previously documented receptor (Kadar et al., 1992, Biochem. Biophs. Res. Comm. 189:289-295).
Although GnRH binding sites have been demonstrated in tumors, such tumors were derived mainly from hormone dependent tissues. Recently, Nechushtan et al. reported that certain hormone non-responsive tumors such as colon carcinomas, renal cell carcinomas and hepatocellular carcinomas were susceptible to killing by a chimeric toxin, GnRH-PE (J. Biol. Chem., 1997, 272:11597). GnRH caused the chimeric toxin to bind to GnRHR-expressing tumor cells, whereas PE mediated cell killing by inhibiting protein synthesis. However, prior to the present invention, it was not known whether the observed effects were due to the expression of a natural GnRHR by hormone non-responsive tumors or a new epitope recognized by GnRH-PE that was distinct from that bound by GnRH.